Formality of Sack Suit vs Business Suit

I have noticed that Emily Post classifies the sack suit as being more formal than the business suit. I thought the the sack was just one of a number of silhouettes that could be used as a business suit. Is it in fact something more than that?

Perhaps you are reading a different edition but this is what I found: http://www.bartleby.com/95/34.html "The business suit or three-piece sack is made or marred by its cut alone." That sentence implies she used the term "sack suit" as the American equivalent of the British "lounge suit".

"The business suit or three-piece sack is made or marred by its cut alone."

That sentence implies she used the term "sack suit" as the American equivalent of the British "lounge suit".

Ah, perhaps she is using the terms interchangeably. The passage that confused me was from the 1955 edition:

"Merely to clear away much confusion: Semi-formal does not mean women in formal evening dresses and men in business suits. In communities where the tail coat is worn, semi-formal means dinner jackets (tuxedos) and simple evening dresses. In others where the dinner jacket is formal, semi-formal would mean men in dark sack suits and women in so-called cocktail dresses."

An incorrect illustration of a "sackcoat" in the 1997 edition showing what seems to be a shawl collar tuxedo didn't help either.

The dinner jacket is, of course, also a type of lounge coat. The older British term in Victorian times was "dress lounge", although this term was superceded in the 19th century by the term "dinner jacket" (see Norah Waugh for the reference). All older American tailoring manuals only use the term "sack coat" to drescribe a lounge coat (just as they always say "cutaway" instead of the British "morning coat"). So perhaps the characterisation of the dinner jacket as a "sackcoat" is also intentional.